Harris’ challenge, like that of Hitchens (“Tell me some good that a religious person can do that an atheist can’t”), seems clever prima facie, but loses its punch when you stop and consider it.
First, I present to you this list of ridiculous theories once believed and posited by scientists, often backed up with real data and evidence that was just missing major pieces. So yes, even scientists can be wrong. But what’s important is not that these ideas were supplanted by a better, more “religious” claim. It is that religion and science coexist, each working in its respective place of supernatural and natural. And in the background, in the code, in the fabric of all natural order and observation, lies a trove of mysteries unanswered — and unanswerable, if the questions traffic in the philosophical or metaphysical realm — by natural sciences. Heck, we barely understand gravity!
These effects — where there are basically curves, hills and valleys in space — occur for reasons we can’t fully really explain. Besides being a characteristic of space, gravity is also a force (but it is the weakest of the four forces), and it might be a particle, too. Some scientists have proposed particles called gravitons cause objects to be attracted to one another. But gravitons have never actually been observed. Another idea is that gravitational waves are generated when an object is accelerated by an external force, but these waves have never been directly detected, either.
So I would answer Harris’ question with something akin to, “Natural sciences cannot answer the oughts and the whys of life; instead, supernatural theories alone stand a chance.”